In the manufacturing process of semiconductor devices, as an apparatus for detecting defects in a pattern formed on a wafer 13, a mirror projection inspection apparatus is known, wherein defects are detected by the following step: applying a sheet electron beam on a semiconductor wafer, forming an image of the electron beam drawn back on the surface of the wafer 13 (hereafter referred to as a mirror electron), and comparing the images for the same pattern of different areas. (See, for example, JP11-108864A.)
Here, a mirror projection inspection apparatus will be described with reference to the FIG. 4. FIG. 4 is a schematic drawing of a mirror projection inspection apparatus comprising a cathode 8, a condenser lens 9, a beam separator 10, an objective lens 7, a projection lens 11, an image detecting unit 12, a charge control unit 14, a wafer 13, retarding power supply 18, an image processing unit 17, a lens control unit 15, and a stage control unit 16.
The retarding power supply 18 applies a voltage to the wafer 13 so that the sheet electron beam applied to the wafer 13 is drawn back in the vicinity of the surface of the wafer 13; The lens control unit 15 controls the lens so that the image detecting unit 12 detects the electron beam drawn back in the vicinity of the surface of the wafer 13; and, the stage control unit 16 moves the wafer 13 in synchronization with the capture of images at the image detecting unit 12 so that the sheet electron beam illuminates throughout the inspection area of the wafer 13; the image processing unit 17 identifies defective portions by sequentially comparing the images captured at the image detecting unit 12 for the same patterns lying side-by-side.
This system realizes a substantial improvement in speed of inspection in comparison with a SEM (Scanning Electron Microscopy) type inspection apparatus.
The process of detecting electrical defects (for example open defects) formed on the wafer 13 by a conventional mirror projection inspection apparatus will be illustrated with reference to the attached drawing. FIG. 2(a) schematically illustrates how the mirror electron 1 is formed into an image on the detection plane 3 by an imaging lens 2: the potential of the wafer 13 and the imaging lens 2 are adjusted so that the mirror electron 1 drawn back on the defective portion 5 is formed into an image on the detection plane 3 as a clear contrast. FIG. 2(b) shows an electron density distribution on the detection plane 3, where the mirror electron 1 drawn back on a defective portion 5 converges on the detection plane 3, and the electron drawn back on a normal portion 4 diverges at the detection plane 3. As the result, an image of the defective portion 5 is formed as a bright spot on the detection plane 3. FIG. 3 shows the case where a defective portion 5 is charged to a different potential from the defective portion 5 in FIG. 2, wherein, on the defective portion 5 in FIG. 3, an electric field distribution 6 different from that in FIG. 2 being formed, the mirror electron 1 does not converge on the detection plane 3. FIG. 3(b) shows an electron density on the detection plane 3, wherein, there being no difference in electron density between the defective portion 5 and the normal portion 4, no image of a defect can be formed there. This is also the case with shape defects, when conditions are set for the mirror electron from a targeted defective portion 5 to converge on the detection plane 3, a mirror electron from a defective portion 5 of different size will not converge on the detection plane 3 and it can not be detected. Therefore, when an inspection is conducted by using a conventional mirror projection inspection apparatus, a defect of different size or of different potential from a targeted defect can not detected.